Let's do some math ..  Rounding examples of 7 random hypothetical purchases:
  Case Tag  
Price  
Sales  
Tax %  
Raw Total  
Incl. Sales Tax  
Rounded  
to Penny  
Rounded  
to Nickel  
Rounded  
to Dime  
  Purchase #1 $249.84   12.849%   $281.9419   $281.94   $281.95   $281.90  
  Purchase #2 $186.37   9.302%   $203.7061   $203.71   $203.70   $203.70  
  Purchase #3 $83.76   8.499%   $90.8788   $90.88   $90.90   $90.90  
  Purchase #4 $19.70   12.629%   $22.1879   $22.19   $22.20   $22.20  
  Purchase #5 $152.74   4.105%   $159.01   $159.01   $159.00   $159.00  
  Purchase #6 $250.76   10.330%   $276.6635   $276.66   $276.65   $276.70  
  Purchase #7 $8.85   4.764%   $9.2716   $9.27   $9.25   $9.30  
  7 purchase totals = $952.02       $1,043.6598   $1,043.66   $1,043.65   $1,043.70